PROJECT SUMMARY/ABSTRACT Atrial fibrillation (AF) is the most common arrhythmia in older women and results in a higher risk of stroke, death and other comorbidities. Current treatments for AF are limited and novel preventive strategies are needed. The effects of physical activity on AF are controversial. We and others have reported reduced rates of AF associated with physical activity, but these studies are based on retrospective observational data. There remains concern that patients at high risk for AF will have worse rates of AF with exercise. Since many older patients are found to have asymptomatic AF, there is also concern that physical activity will result in silent AF that is not detected with standard clinical care. Our long term goal is to reduce rates of AF with safe and appropriate lifestyle recommendations. The WHI Strong & Healthy (WHISH) trial is an NIH-funded study where women will be randomized to receive physical activity intervention that includes exercise instructions via mailings and an interactive voice response system and followed for subsequent cardiovascular outcomes. We propose an ancillary study (WHISH STAR) designed to take advantage of the infrastructure established by WHISH and to add a cardiac monitoring component to detect silent AF in a subset of women at high risk for AF. The primary purpose of WHISH STAR is to study the effects of PA on atrial fibrillation. Our central hypothesis is that physical activity intervention reduces rates of clinical and silent AF. We will achieve our objective by addressing the following aims: 1) To study the effect of physical activity intervention on standard clinical AF in older women. Women without baseline AF will be followed for standard, clinical AF with adjudicated chart reviews and Medicare linkage data. 2) To study the effect of physical activity intervention on silent AF in women with high AF risk. Women at high risk of developing AF, defined as a 10-year Framingham risk of ? 12% will undergo 14-day cardiac patch ECG monitoring at baseline, 6 months and 1 year. 3) To determine if changing from a sedentary state to an active state results in a decrease in AF risk. This proposal is innovative because a large prospective physical activity intervention trial to study the effects on atrial fibrillation has never been done and this study leverages the infrastructure of the parent WHISH trial, which will minimize funds needed for consent or outcome assessment, but adds significantly to the existing study by including validation of outpatient episodes of AF. The team assembled has extensive experience in the ascertainment of AF in the WHI, expertise in clinical trial design, insight into the epidemiology of cardiac screening, statistical expertise and the organizational tools necessary to successfully complete the study. This study will be significant because it will have an immediate impact on the advice regarding physical activity that is given to older populations at risk for AF and will help determine if moving subjects out of a sedentary state could have a particularly strong clinical benefit.